Chapter 9 ENTHALPY CHANGES

• What is enthalpy?

  A measure of the heat energy in a chemical system where the chemical system refers to atoms, molecules or ions making up the chemical
• How do you calculate enthalpy change ($\Delta H$)?


  Enthalpy of products - enthalpy of reactants
• What is conservation of energy?

  Energy cannot be created or destroyed, only transferred or transformed from one form to another.
• What is an exothermic reaction?

  Energy is transferred from the system to the surroundings
• What is $\Delta H$ for exothermic reactions?


  Negative
• What is an endothermic reaction?

  Energy is transferred from the surroundings to the system
• What is $\Delta H$ for endothermic reactions?


  Positive
• What is activation energy?

  The minimum energy required for a reaction to take place and break bonds between the reactants
• What does a small activation energy mean?

  The reaction progress will happen rapidly as the energy needed to break bonds is readily available
• What are standard conditions?

  100kPa, 1 moldm-3, 298K, standard state
• What is the enthalpy change of reaction ($\Delta rH\theta$)?


  The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states
• What is the enthalpy change of formation ($\Delta fH\theta$)?


  The enthalpy change that takes place when one mole of a compound is formed from its constituent elements under standard conditions, with all reactants in their standard states
• What is important to remember when writing an equation for formation?

  MUST not use whole balancing numbers as it wont comply with the definition for enthalpy change of formation
• What is the enthalpy change of combustion ($\Delta cH\theta$)?


  The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states
• What is important to remember when writing a combustion equation?

  MUST NOT be balanced fully otherwise doesn't obey the definition for enthalpy change of combustion
• What is the enthalpy change of neutralisation ($\Delta neutH\theta$)?


  The enthalpy change that accompanises the reaction of an acid by a base to form one mole of water under standard conditions, with all reactants and products in their standard states
• How is energy change calculated?

  $q=$$mc\Delta T$ where m is the mass, c is the specific heat capacity and $\Delta T$ is the change in temperature
• How can a spirit burner be used to determine energy change of liquid fuels?

  1. Add set volume water to beaker and record temperature
  2. Add the fuel and record weight of the burner + fuel
  3. Place burner under beaker and heat
  4. Weigh burner after and record new temperature
  5. Calculate energy change
• Why is the data book value of $\Delta cH$ more exothermic than the one achieved using the spirit burner?


  Heat loss to surroundings, incomplete combustion, evapouration of fuel from wick, non-standard conditions
• What experiment can be used to determine the enthalpy change of reaction?

  Polystyrene cup experiment with a solid and a solution
• How is the polystyrene cup experiment done?

  1. Excess solid weighed into polystyrene cup with solution
  2. Temperature measured before + after reaction takes place
  3. Energy change calculated using $q=$$mc\Delta T$
• How can the polystyrene cup experiment be used to calculate enthalpy change of neutralisation?

  Using 2 solutions instead of a solid and a solution
• What is average bond enthalpy?

  The energy required to break one mole of a specified type of bond in a gaseous molecule
• Is bond breaking exo or endothermic?

  Endothermic
• Is bond making exo or endothermic?

  Exothermic
• Why are bond enthalpies aways endothermic?

  Energy is always required to break bonds
• What is Hess' Law?

  Enthalpy change of a reaction is the sum of the enthalpy changes of its steps
• Which way do arrows go in a Hess cycle if $\Delta fH\theta$ is given?


  From the constituent elements TOWARDS the products of the reaction
• Which way to arrows go in a Hess cycle if $\Delta cH\theta$ is given?


  From the reactants DOWNWARDS to the combustion products